The use of In-Situ Lift-Out (INLO) for TEM sample preparation in the FIB has become a popular and widely accepted technique. INLO enables the preparation of multiple site-specific TEM samples, at different angles of inspection, and with the imaging resolution of a Scanning Electron Microscope (SEM), without the need for an expensive wet lab for conventional sawing, polishing and grinding, and without the need to sacrifice the wafer being inspected. The ability to perform process control on 300 mm diameter wafers without sacrificing wafers for the inspection is very important because of the value of these wafers.
However, accurate process control requires high throughput TEM sample preparation. Automation of the sample preparation process will significantly and favorably impact the analytical throughput of this process and its repeatability.
A key apparatus for INLO is an in-situ nano-manipulator that enables full wafer analysis, such as the AutoProbe 200™ manufactured by Omniprobe, Inc. This nano-manipulator can be used to lift-out a tiny wedge-shaped portion (typically 5×5×10 μm) of the sample and to transfer it to a TEM sample holder that is also present in the FIB vacuum chamber.
Ion or electron-beam assisted deposition of metal or other materials from appropriate source gases injected near the surface in the FIB can be used to attach the nano-manipulator probe tip to the excised lift-out sample. The same beam-assisted gas chemistry can be used to attach the lift-out sample to the TEM sample holder. Later, the ion beam in the FIB can be used to detach the probe tip from the lift-out sample, completing the in-situ transfer of the lift-out sample to the TEM sample holder. This lift-out sample can then be thinned to an appropriate thickness for TEM inspection (<100 nm).
Surface contact detection is a critical element of the automation of such a nano-manipulator-based operation. One of the methods that can be used to determine that the contact between the nano-manipulator probe tip and the sample surface has been made is electrical continuity detection. This method is impractical for automation due to several reasons. If the sample surface is non-conductive, the detection of the steady-state electrical continuity between the probe tip and the sample surface will not be successful. Even if the sample surface is electrically conductive, it may not be electrically connected to the sample holder, or there may be a tough native oxide on the conductive surface making continuity detection difficult. Without an electrical connection to the sample holder, continuity detection between the probe tip and sample surface will be difficult in the FIB environment. Detection of a transient electrical response due to the connection of the charged sample surface with the probe tip is also impractical for repetitive automated procedures because this effect is time and material dependent and also depends on the behavior of the charged particle beams impinging the surface.
A metal layer that covers the surface of the sample and electrically connects the surface and the sample stage may be deposited using an appropriate gas source and electron or ion beam assisted deposition in the FIB. Such a deposition operation is time consuming in the FIB, however, and may render the wafer useless for further processing. In addition, the ion beam may locally remove the metal layer at the place where the probe tip makes contact with a sample surface and hence defeat the purpose of the inspection.
What is needed is a safe and reliable method of detecting contact between the sample and the probe of the nano-manipulator, whether inside or outside of a FIB.